Wish upon a shooting star - the Geminids meteor shower peaks tonight
12 December 2001Meteor watchers in Europe may be compensated for missing last month's splendid Leonid meteor storm when the annual Geminids shower peaks in the early hours of Friday morning. Up to 100 shooting stars per hour may be visible if observing conditions are good.
The Geminids meteor shower is an annual event triggered by the passage of Earth through the outskirts of a dusty debris cloud thrown off by a rocky asteroid called 3200 Phaethon.
Although nowadays regarded as one of the best meteor showers of the year, this was not always the case. Prior to the 1860's there were no recorded sightings of Geminids. Their appearance seems to have been fairly sudden - the first shower was recorded in 1862, surprising sky watchers who saw 15 or so unexpected shooting stars each hour. Those early showers were unimpressive, but since then the Geminids have grown in intensity.
"Generally speaking, this shower is not as intense as the 2001 Leonids shower," says ESA scientist Detlef Koschny, "but compared to what was visible here in Europe in November, this will be more spectacular. The night is also going to be dark: no Moon will disturb the observations."
Most meteor showers are an effect of debris that boils off a comet's nucleus when it passes close to the Sun. This debris then begins to orbit, along with the parent comet, forming a stream of meteoroids that become shooting stars when they hit Earth's atmosphere.
"The case of the Geminids, however, is a bit peculiar," says Detlef Koschny. "This is one of the very few cases known where the meteor trail results from debris left by an asteroid instead of a comet. The rocky asteroid, called 3200 Phaethon, was discovered in the 1980's. This unusual parent body means that, even if the difference is not visible to the naked eye, scientists will be able to distinguish the particles generating the shooting stars: they will not be fluffy, but rather stony."
Scientists have a few ideas to explain the odd phenomenon of an asteroid forming a debris tail. One explanation could be that there was an impact with a smaller body which produced the dust. Another explanation is that asteroids and comets are perhaps just two extremes of the same phenomenon: when it has more ice, we call it a comet; when there are more silicates (material containing silicon and oxygen), we call it an asteroid. Perhaps Phaethon's ice has vapourized and the Geminids are the leftovers of a former comet.
Whatever the explanation, ESA's Rosetta mission, due to be launched in 2003, will shed more light on this hypothesis with its remote observations of Comet Wirtanen and its in-situ investigation of Wirtanen's nucleus.
In the meantime, for those brave enough to challenge the rigours of a winter night, the shooting star show begins tonight, after sunset. The best chance of seeing meteors is in the morning hours before dawn.